I've had this EXACT. SAME. ISSUE. Multiple times over the x years I've played LoL. I just had it in a ranked match that was close, but we were winning in. It is things like this which make people rage and plot the destruction of the masses. =p
Still pretty angry about it, it cost me a loss after something like 10~12 Voli wins in a row and getting into promos.
I was using WiFi tether, getting about 37 megs download and 20 megs upload speed (yeah, it's amazing, I know), when it started intermittently happening. My ping/latency was fine, all other forms were working fine (Skype, did a quick YouTube check, checked RDP/RemoteDesktop, etc), and my traceroute didn't go above 100 ms at any hops. Please keep in mind I'm in the Los Angeles area. I swapped to my office WiFi as well as ethernet cable and the game would not respond at all still.
I've not had this issue in any other games, be it Don't Starve Together, Heroes of the Storm, Starbound, Rust, Diablo 3, World of Warcraft, etc. If I ever had a connection issue, it would always be network-wide, and not ONLY be targeting one game. What can I traceroute specifically as an IP for NA game servers in the future to show any issues that will help a Dev team, NOC team, or any other team sort this out? It's cost me wins in the past and sadly, nothing leaves a worse taste in a player's mouth than when they're doing well overall and they come up dry due to something outside of their control to the point of usually turning to raging/negativity/outright quitting a game.
I do not say such things as a tantrum of sorts, I am simply trying to convey that when these types of things happen, players will generally leave a game for awhile/permanently if it's unplayable. I've not experienced this in about 9 months, and it just began happening again from an office in Sherman Oaks which is just a hop, skip, and a jump from Riot HQ. The last time it happened a lot, I was playing from my office in Marina del Rey where I had 0 ping as Riot was just up the street. And it still happened.
So I know for sure it isn't my setup/configuration, nor is it due to any form of firewall on my end, etc. Could someone PLEASE tell me what causes others to move/function fine and my champion sitting there stationary? I click, it registers the sound, and nothing happens. Skill keys don't happen. I can still type and send messages. Thank you.